JP4338275B2 - Miller cycle engine operation method and exhaust manifold volume setting method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention cools the supply air pressurized by the supercharger with the cooler and then supplies the air into the cylinder through the supply port, and lowers the supply valve provided on the cylinder supply port side. The present invention relates to a delayed closed mirror cycle engine that closes later than the point and blows back part of the air supply in the cylinder back to the air supply port to lower the compression ratio than the expansion ratio. The present invention relates to an engine that is expected to be used as a mirror cycle gas engine.
[0002]
[Prior art]
A mirror cycle is known as a means for improving the fuel consumption of an internal combustion engine.
The Miller cycle is an effective expansion ratio that achieves high thermal efficiency while avoiding knocking by keeping the compression ratio smaller than the expansion ratio, and sufficiently expands the energy of the combustion gas. The heat efficiency is improved.
Combining this mirror cycle with lean combustion enables further expansion of the expansion ratio and further increases in thermal efficiency.
[0003]
[Problems to be solved by the invention]
However, when such a lean burn mirror cycle is applied to an actual engine, the following problems occur.
In the application of the mirror cycle to the engine, the expansion ratio of the engine is made larger than the compression ratio. This includes a method of increasing the engine stroke to increase the cylinder volume at the end of expansion and a method of reducing the cylinder volume at the start of compression while keeping the engine stroke constant.
The former method is not practical because the cylinder volume becomes extremely large and the engine per unit output becomes too large. Therefore, the latter method is often used in practice. In this case, in order to maintain the output of the engine, it is necessary to supply a high-density air-fuel mixture to a small cylinder volume, and it is necessary to supply an air-fuel mixture previously highly compressed by a supercharger. Generally, an exhaust turbine supercharger is generally used for the supercharger, and increasing the compression work of the air-fuel mixture in this way is nothing but increasing the expansion work in the supercharger turbine.
Further, in order to increase the expansion work as compared with the theoretical air ratio engine, it is necessary to increase the exhaust turbine inlet pressure. In a lean combustion engine, since the exhaust gas temperature is low, in order to obtain a large expansion work for a small amount of exhaust gas energy, the exhaust turbine inlet pressure of the engine must be increased.
That is, the lean burn Miller cycle engine tends to have a large pumping loss as compared with the conventional engine or the theoretical air ratio Miller cycle engine. For this reason, it is difficult to sufficiently secure the effect of improving the efficiency of the mirror cycle engine by increasing the thermal expansion ratio.
[0004]
This situation is described below based on the PV curve of the engine.
FIG. 6 shows a PV curve of a general turbocharged gas engine. Since the hatched portion in FIG. 6 represents positive work and the shaded portion represents negative work (pumping loss), the work amount of the gas engine is obtained by subtracting the shaded portion from the shaded portion. An engine with good thermal efficiency means an engine having the same fuel consumption and a large area of a hatched portion minus a shaded portion.
FIG. 7 shows a PV curve of a low pressure portion of a lean combustion gas engine that is not a mirror cycle. In this case, the turbocharger turbine inlet pressure can be kept low for the reasons described above, so that the cylinder pressure during the exhaust stroke of the engine is low and the pumping loss is kept small.
On the other hand, in the lean combustion mirror cycle, the compression work and expansion work of the supercharger are large, and the exhaust gas temperature is low, so the supercharger turbine inlet pressure tends to be high.
In addition, since the turbocharger pressure ratio is large and the turbocharger cannot be used at a point with a low efficient pressure ratio, the turbocharger efficiency decreases, and the exhaust turbine inlet pressure of the engine becomes even higher. If the engine structure shown in FIG. 7 is applied as it is to obtain a lean mirror cycle, the pumping loss increases as shown in FIG. That is, as described above, the efficiency of the mirror cycle engine cannot be sufficiently exhibited.
[0005]
Therefore, although a concept combining a mirror cycle engine and lean combustion has been conventionally known, a lean combustion mirror cycle engine has not yet been put into practical use.
An object of the present invention is to obtain a method for operating a lean combustion mirror cycle engine and a method for setting an exhaust manifold volume, which can suppress a pumping loss to a reasonable range in a mirror cycle engine that performs lean combustion, and is advantageous in terms of thermal efficiency. It is in.
[0006]
[Means for Solving the Problems]
In solving the above problems, the inventors have considered reducing the cylinder pressure during the exhaust stroke of a lean burn mirror cycle engine to improve the fuel efficiency of the engine. More specifically, in a lean burn mirror cycle engine, by optimizing the relationship between the closing timing of the intake valve and the excess air ratio and the volume of the exhaust manifold, the pumping loss of the engine can be reduced and the engine efficiency can be reduced. The inventors found that it can be improved and completed the present invention.
The operating condition range of the engine that can exert a great effect as a lean combustion mirror cycle is that the excess air ratio is 1.5 or more and the supply valve closing timing reference compression ratio is 0.7 times the stroke volume reference compression ratio, as will be described later. The range is as follows, and the engine is preferably used in such a range.
Here, the stroke volume reference compression ratio is the cylinder internal volume V2 / when the piston is at the top dead center / cylinder internal volume V1 when the piston is at the bottom dead center, and the supply valve closing timing reference compression ratio is: It is defined as cylinder internal volume V2 when the piston is at top dead center / cylinder internal volume V3 at the time when the intake valve is closed late.
Further, the excess air ratio γ is defined as the air-fuel ratio A / F with respect to the theoretical mixture ratio.
In the present application, the volume of the exhaust manifold means the volume from the outlet of each cylinder of the engine to the inlet of the turbocharger.
The ratio between the lean combustion configuration of the mirror cycle gas engines, the ratio of the volume and the engine total engine exhaust manifold t, Kyukiben閉timing reference compression ratio and the stroke volume reference compression ratio in order to achieve the above object Where u is the air excess ratio and γ is the excess air ratio, the supply valve closing timing reference compression ratio is 0.7 times or less of the stroke volume reference compression ratio, the excess air ratio γ is 1.5 or more, and the exhaust manifold volume is , (1 / u × γ) ^0.5 / 10 ≦ t ≦ 3.
If the exhaust manifold volume satisfies the above requirement in the relationship between u and γ, the pumping loss that is a problem in the lean combustion mirror cycle can be suppressed to a state close to its lower limit value.
Here, when t is smaller than the lower limit, the pumping loss increases rapidly, which is not preferable. On the other hand, if t is greater than 3, the exhaust manifold becomes too large, which is practically expensive and not practical.
By setting the relationship between the two compression ratios and the excess air ratio to the above values, it is possible to achieve an operation state in which the engine efficiency can be maintained at a relatively high level. By doing so, higher efficiency can be realized. Here, the lower limit of the supply valve closing timing reference is about 4, and the upper limit of the excess air ratio is about 2.5.
Eventually, after the supply air pressurized by the supercharger is cooled by the cooler, the air is supplied into the cylinder through the supply port, and the supply valve provided on the supply port side of the cylinder is dead. The operation method of the Miller cycle engine which closes later than the point and blows back a part of the air supply in the cylinder to the air supply port to lower the compression ratio than the expansion ratio is described in claim 1. The ratio of the supply valve closing timing reference compression ratio and the stroke volume reference compression ratio is u, the excess air ratio is γ, the ratio of the exhaust manifold volume and the total engine displacement is t,
A Miller cycle engine satisfying (1 / u × γ) ^0.5 / 10 ≦ t ≦ 3, the air supply valve closing timing reference compression ratio is 0.7 times or less of the stroke volume reference compression ratio (u ≦ 0.7), air It is preferable to operate under an operating condition where the excess rate γ is 1.5 or more from the viewpoint of reducing pumping loss and, in turn, improving engine efficiency.
[0007]
As described in claim 2, such a range of t is such that the ratio t between the volume of the exhaust manifold and the total engine displacement is (1 / u × γ) ^0.5 /6≦t≦0.8. Can be set within the range.
The lower limit of t is a lower limit value that substantially realizes the lower limit of the pumping loss that can be reached under a predetermined engine operating condition when the exhaust manifold volume is increased. By setting this value as the lower limit, A lean burn mirror cycle can be realized with sufficiently reduced loss. Further, by setting t to about 1, the exhaust manifold volume can be maintained in a state that does not become excessive.
Further, when the exhaust manifold is set in the above-described range, the volume of the exhaust manifold increases as the air supply valve is gradually closed. This means that the larger the turbocharger work, the larger the exhaust manifold volume. This reduces the pressure fluctuation at the turbocharger turbine inlet, thus improving the efficiency of the turbocharger and Efficiency can also be improved.
[0008]
Furthermore, the ratio t between the volume of the exhaust manifold and the total engine displacement is more preferably set in the range of (1 / u × γ) ^0.5 /6≦t≦0.6. The lower limit of t is based on the above-mentioned reason, but the upper limit is based on practicality.
[0009]
Thus, in the method of setting the exhaust manifold volume with respect to the total engine displacement, as described in claim 3, the range of the ratio t between the exhaust manifold volume and the total engine displacement is supplied. It is preferable to set based on the reciprocal 1 / u of the ratio u between the air valve closing timing reference compression ratio and the stroke volume reference compression ratio and the engine air excess ratio γ.
In such a lean combustion mirror cycle, the pressure at the exhaust manifold portion depends on the above 1 / u and γ, and therefore, the exhaust manifold volume is determined in consideration of these requirements. When realizing a lean combustion mirror cycle, the pumping loss is set to a sufficiently low state, and an engine capable of achieving an improvement in efficiency is obtained.
As described above, it is preferable to set the exhaust manifold volume with respect to the total engine displacement by the method as described in claim 4 above, but in this case as well, as described in claim 1 In order to set the ratio t between the exhaust manifold volume and the total engine displacement, it is preferable that (1 / u × γ) ^0.5 /6≦t≦0.8 is satisfied. The reason for this is the same as described above.
This mirror cycle is particularly effective for a cogeneration engine that is operated at a constant load and a constant rotation speed due to the complexity of control, but in a diesel engine, by controlling the fuel injection timing and injection amount, Since knocking can be avoided, applying a mirror cycle does not provide a significant effect. For this reason, it is particularly effective to employ an engine structure capable of realizing a lean combustion mirror cycle as in the present application, particularly in a cogeneration gas engine.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
A flow of the turbocharged gas engine 1 is shown in FIG. In the air supply system, the air-fuel mixture mixed by the mixer 2 is compressed by the blower unit 3 of the supercharger 10. The air-fuel mixture that has become hot due to compression is cooled by the air supply cooler 4, then led to the air supply port 5 and supplied into the cylinder 6 of the gas engine 1. The exhaust gas is exhausted after passing through the turbocharger turbine section 7 and driving the supercharger 10.
When the engine 1 is operated, the supply air pressurized by the supercharger 10 is cooled by the intake air cooler 4 and then supplied to the cylinder 6 through the supply air port 5. The air supply valve 50 provided on the port side is closed with a delay from the bottom dead center, and a part of the air supply in the cylinder 6 is blown back to the air supply port 5 so that the compression ratio is lower than the expansion ratio. . In this case, the supply valve closing timing reference compression ratio is 0.7 times or less (preferably 0.7 or less 0.3 or more) of the stroke volume reference compression ratio, and the excess air ratio γ is 1.5 or more (preferably 1.5 to 2.5).
[0011]
Now, the change of the pumping loss was examined by changing the volume of the exhaust manifold for the mirror cycle gas engine having the above-described configuration. For this investigation, we conducted experiments with actual machines and numerical simulations.
The results are shown in FIG. Hereinafter, the experimental situation and the numerical simulation situation will be described in order.
[Experiment]
In a state where the engine speed is 1200 rpm and the excess air ratio γ is 2, the exhaust manifold volume is changed to 0.1 and 0.4 by the exhaust manifold volume / total engine displacement, and the supply valve closing timing The ratio u between the reference compression ratio and the stroke volume reference compression ratio was changed in the range of 0.4 to 0.7.
The main requirements for engines other than these are as follows.
Main fuel City gas 13A,
Total stroke volume 4086 x 6cc,
Number of cylinders 6, inner diameter x stroke (mm) 170 x 180,
Supply valve open / close timing opened 38 ° deg BTDC,
closed 45-116 ° ABDC
Exhaust valve opening and closing timing opened 58 ° deg BBDC,
closed 25 ° deg ATDC,
The supply air temperature at the outlet of the aftercooler was about 45 ° C., and the supply air pressure was about 9.8 × 10 ⁴ Pa to 2.45 × 10 ⁴ Pa.
The results are shown by circles in FIG.
[Numerical simulation]
Corresponding to the above experiment, the pumping loss for the change in the volume of the exhaust manifold when the engine speed, excess air ratio γ, etc. correspond to the above conditions and u is the same value as above. Asked.
The result of the simulation is shown by the solid line in FIG. Corresponding to each line, u as a parameter is shown. u was also changed from 0.4 to 0.7.
In the figure, the volume of the exhaust manifold is shown as a multiple of the total engine displacement. From this figure, it can be seen that when the volume of the exhaust manifold is increased, the pumping loss is reduced and a lower limit value exists.
The situation shown in FIG. 2 can be explained as follows.
The required exhaust manifold size is approximately proportional to (1 / u) ^0.5 . Further, 1 / u is a parameter indicating the supply valve closing timing. The larger this value, the higher the supply air pressure. As the late closing progresses, a higher air supply pressure is required, so that the cylinder pressure during the engine exhaust stroke inevitably increases and the pumping loss increases. In order to reduce the pressure in the cylinder during the engine exhaust stroke, it is better that the pressure in the exhaust manifold is constant. For this purpose, a large exhaust manifold is required. On the other hand, in order to increase the excess air ratio γ, a high air supply pressure is still necessary, and the same tendency is observed.
In other words, the size of the exhaust manifold necessary for improving the efficiency is approximately proportional to (1 / u × γ) ^0.5 . Therefore, it is preferable to set the lower limit value of the exhaust manifold volume according to this index.
Further, as can be seen from the numerical calculation results of FIG. 2, the pumping loss when u is kept constant decreases relatively early in the region where t is small as the exhaust manifold volume increases, but t is a certain value. This reduction does not occur if the range is exceeded. That is, the line indicating the pumping loss in FIG. 2 is a line parallel to the horizontal axis.
Thus, increasing the exhaust manifold volume to a region where the pumping loss cannot be reduced only leads to an increase in the size of the engine.
Therefore, as shown in FIG. 2, the lower limit of t is (1 / u × γ) ^0.5 / 10 (shown by a broken line) or (1 / u × γ) ^0.5 / 6 (shown by a two-dot chain line). can do. Here, the former limit is a limit at which a sign of change appears in the decreasing trend, and the latter limit is a limit at which no improvement accompanying an increase in the exhaust manifold volume is observed. As for the upper limit of the exhaust manifold volume, about three times the engine displacement is a limit for practical reasons.
The experimental results and numerical simulation results are in good agreement.
[0012]
[Experiment]
FIG. 3 shows an example of the present invention, where the ratio of the compression ratio is 0.4, the excess air ratio is 2, and the engine speed is 1200 (γpm). The PV of the low pressure part of the engine when it is 4 times (t = 0.4) is shown. It can be confirmed that the pumping loss of the engine is reduced as compared with the conventional example (t = 0.1) shown in FIG.
The pumping loss can be reduced to about 2/3 of the conventional example. (In an engine having an average effective pressure of 110 × 10 ⁴ Pa, the pumping loss is improved from 5.9 × 10 ⁴ Pa to 3.9 × 10 ⁴ Pa, and the engine efficiency is improved by 0.7 to 0.8%. ).
[0013]
On the other hand, FIG. 4 shows the relationship between the compression ratio ratio u and the engine shaft end efficiency. When u is increased, the pumping loss is reduced as shown in FIG. 4, but the engine shaft end efficiency is deteriorated. This is because by increasing u (advance the closing timing of the intake valve), the original advantage of the Miller cycle engine of compression ratio <expansion ratio is weakened and knocking occurs. Cannot be realized. If the air supply valve closing timing is further advanced, this tendency seems to be further strengthened, and the advantage of the mirror cycle cannot be exhibited.
That is, when the effect as a mirror cycle is expected, it is appropriate to set u to 0.7 or less. Furthermore, when this value becomes lower than 0.4, the tendency of decreasing efficiency appears again. In this experiment, following the conditions of the experiment described above, the engine speed was set to 1200 rpm and the excess air ratio was fixed to 2.
[0014]
FIG. 5 shows the relationship between the excess air ratio γ and the engine shaft end efficiency. If the excess air ratio γ is too small, knocking occurs, so that good combustion cannot be achieved, and it is necessary to ensure an excess air ratio of 1.5 or more. The engine speed was fixed at 1200 rpm and u = 0.4. Here, the upper limit of the excess air ratio γ is 2.5 for reasons of stability of combustion (combustion becomes unstable when it becomes too lean).
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of a Millercycle gas engine. FIG. 2 is a diagram showing a relationship between an exhaust manifold volume / engine displacement t and a pumping loss. FIG. 3 is a graph of an engine with an exhaust manifold that satisfies the requirements of the present application. FIG. 4 is a graph showing the relationship between the supply valve reference compression ratio / stroke volume reference compression ratio u and the engine efficiency. FIG. 5 is a graph showing the relationship between the excess air ratio γ and the engine efficiency. Illustration of loss [Fig. 7] PV diagram of a lean combustion gas engine that is not a mirror cycle [Fig. 8] PV diagram when the engine shown in Fig. 7 is operated as a mirror cycle [Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Lean combustion mirror cycle engine 2 Mixer 3 Supercharger blower 4 Air supply cooler 5 Air supply port 6 Cylinder 7 Supercharger turbine 10 Supercharger 50 Air supply valve

Claims (4)

After the supply air pressurized by the supercharger is cooled by the cooler, the air is supplied into the cylinder through the supply port, and the supply valve provided on the supply port side of the cylinder is A method for operating a Miller cycle engine in which a compression ratio is lowered than an expansion ratio by delaying and closing and blowing a part of the supply air in a cylinder back to the supply port,
Assuming that the ratio between the air valve closing timing reference compression ratio and the stroke volume reference compression ratio is u, the excess air ratio is γ, and the ratio between the exhaust manifold volume and the total engine displacement is t,
A Miller cycle engine satisfying (1 / u × γ) ^0.5 / 10 ≦ t ≦ 3, the air supply valve closing timing reference compression ratio is 0.7 times or less of the stroke volume reference compression ratio (u ≦ 0.7), air A method for operating a mirror cycle engine that operates under an operating condition where the excess rate γ is 1.5 or more. The method for operating a Miller cycle engine according to claim 1, wherein a Miller cycle engine satisfying (1 / u x γ) ^0.5 / 6 ≤ t ≤ 0.8 is used.   After the supply air pressurized by the supercharger is cooled by the cooler, the air is supplied into the cylinder through the supply port, and the supply valve provided on the supply port side of the cylinder is A method for setting the exhaust manifold volume of a mirror cycle engine that lowers the compression ratio rather than the expansion ratio by closing and delaying and returning a part of the air supply in the cylinder back to the air supply port. Is set based on the reciprocal 1 / u of the ratio u between the supply valve closing timing reference compression ratio and the stroke volume reference compression ratio and the excess air ratio γ. 4. The mirror cycle engine according to claim 3, wherein (1 / u × γ) ^0.5 /6≦t≦0.8 is satisfied in setting the ratio t between the volume of the exhaust manifold and the total engine displacement. How to set the exhaust manifold volume .

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